The present invention relates to constant current power supplies and more particularly to constant current power supplies capable of delivering high frequency current to a poorly matched load.
In U.S. Pat. No. 4,256,945 of Carter and Krumme, there is described an auto regulating electric heater having a laminated structure; one lamina of which has high magnetic permeability and high resistance and another lamina of which is non-magnetic and has a low resistance (such as copper) in electrical contact, and therefore, thermal contact with the first lamina. This structure is adapted to be connected across a constant current, a.c., source such that the layers are in a sense in parallel across the source.
Due to skin effect, the current is initially confined to the high magnetic permeability, high resistance layer so that P=KR.sub.1 where P is Power, K is I.sup.2 which is a constant, and R.sub.1 is the effective resistance of the permeable material at high current concentrations. The dissipation of power heats the layer until it approaches its Curie temperature. The permeability of the lamina decreases towards the level of the second layer, copper for instance, at about its Curie temperature. The current is no longer confined to the high resistivity first lamina by the magnetic properties of the first lamina, and spreads into the copper layer; the resistance to the current drops materially, the power consumed, P=KR.sub.2 where R.sub.2 &lt;&lt;R.sub.1, is greatly reduced and the heating effect is reduced to a level that maintains the device at or near the Curie temperature. The device thus thermally auto regulates over a narrow temperature range about the Curie temperature.
The current source employed in the aforesaid patent is typically a high frequency source, to insure that the current is confined to the thin, high resistivity, magnetic layer until the Curie temperature of the magnetic material is attained. Specifically, the maximum regulation is achieved when the thickness of the magnetic layer is of the order of one to 1.8 skin depths at the frequency of operation. Under these circumstances, the maximum change in effective resistance of the structure is achieved at or about the Curie temperature. This fact can be demonstrated by reference to the equation for skin depth in a monolithic, i.e., non-laminar magnetic structure: EQU S.D.=5030.sqroot..rho./.mu..function. cm,
where .rho. is the resistivity of the material in ohm-cms, .mu. is magnetic permeability mu and f is frequency of the current. The field falls off in accordance with e.sup.-x where x is thickness/skin depth. Accordingly, in a monolithic structure, by calculation, 63.2% of the current is confined to one skin depth in the high mu material. In the region of the Curie temperature, where m.mu.=1, the current spreads into a region S.C.=5030 .sqroot..rho./.function. cm. If mu was originally equal to 200 (200-5000 being possible), the skin depth in the region at the Curie temperature increases by the square root of 200; i.e., the skin depth in the monolithic structure is now 14.14 times greater than with .mu.=200.
The same type of reasoning concerning the skin effect may be applied to the two layer laminar structure in the aforesaid patent. Below the Curie temperature, the majority of the current flows in the magnetic layer when the thickness of this layer is nominally one skin depth of the material below the Curie temperature. In the region of the Curie temperature, the majority of the current now flows in the copper and the resistance drops dramatically. If the thickness of this high mu material were greater than two skin depths, the percentage change of current flowing in the high conductivity copper would be less and the resistivity change would not be as dramatic.
Similarly, if the thickness of the high mu material were materially less than one skin depth, the percentage of current flowing in the high resistivity material at a temperature less than the Curie temperature would be less so that the change of resistance at the Curie temperature would again not be as dramatic. The region of 1.0 to 1.8 skin depth is preferred.
An exact relationship for the two layer case is quite complex. The basic mathematical formulas for surface impedance from which expressions can be obtained from the ratio of the maximum resistance, R max, below the Curie temperature, to the minimum resistance, R min, above the Curie temperature, are given in Section 5.19, pp. 298-303 of the standard reference, "Fields and Waves in Communications Electronics," 3rd Edition, by S. Ramo, J. R. Winnery, and T. VanDuzer, published by John Wiley and Sons, New York, 1965. Although the theory described in the above reference is precise only for the case of flat layers, it is still accurate enough for all practical applications in which the skin depth is substantially less than the radius of curvature.
A difficulty is encountered when any of the above heaters are employed in soldering irons such as illustrated in FIG. 4 of the aforesaid patent. The impedance of the soldering iron, due to its relatively small size, is quite low (of the order of 0.1 to 0.25 ohm) and in consequence, presents a poor impedance match to the source. This problem is mitigated to some extent by including impedance matching circuits in the handle of the soldering iron. In such a case, however, a greater resistance appears in the handle than at the tip of the iron, making the handle quite hot and the overall soldering iron performance quite inefficient.
In the aforesaid co-pending application Ser. No. 666,346 for High Efficiency Auto regulating Heater, a heater illustrated as a soldering iron, is described which provides a substantially matched load to the supply below the temperature at which the permeability of the magnetic material begins to fall. Once the permeability begins to degrade, however, the impedance match also degrades reaching a quite poor level, a VSWR of as high as 5 or 6, when the permeability of the magnetic material falls to about 1. With such a VSWR prior art power supplies are subject to destruction.